1. Field of the Invention
The invention relates to the groove contour of the substantially circularly grooved rolls of a reducing mill with three-high rolling stands, especially for rolling thick-walled tubes with a wall thickness/diameter ratio greater than 0.25.
2. Discussion of the Prior Art
In order to roll thin-walled tubes, reducing mills according to the prior art normally do not use circular groove forms. Instead, such reducing mills use elliptical grooves formed by three curvatures, the short semiaxes of which are located in the groove base. The long semiaxis is located in the groove discontinuity and is larger than the short semiaxis of the preceding groove. This measure represents an attempt to prevent the rolled material from emerging into the roll gap and in this way to avoid roll gap markings. However, the technical limits of such groove forms are reached in rolling tubes with wall thickness ratios greater than 0.1. In these cases, so-called "polygon formation" occurs, i.e., the internal contour of the tube deviates from the desired circular shape. In an extreme case, such polygon formation can result in an almost hexagonal internal cross-section, which severely limits the usefulness of the tube.
It is nearly impossible to roll thick-walled tubes using the known oval grooves. Instead, grooves that are more nearly circular in shape and can better suppress polygon formation must be used. These correlations are discussed in the "Archive of Iron and Steel Metallurgy" (Archiv fuer das Eisenhuettenwesen), Vol. 41, No. 11, November 1970, pp. 1047-1053. In conclusion, this known prior art proposes that reducing mills use special grooving for the thick-walled portions of the program of a tube rolling train. The special grooving strives to encompass the tube on all sides during the rolling process, i.e., to achieve a substantially circular groove. It is possible to adopt such a measure because roll gap markings are significantly less likely to appear on the external surfaces of thick-walled tubes than on those of thin-walled tubes. The groove opening should be selected so the difference in length between the shortest and longest contact areas is minimal, i.e., in such a way as to create a rectangular contact surface between the roll and the tube.
When the principle stated in the cited publication is applied, however, it has been shown that an even contact surface cannot be achieved between the tube and the roll in pass sequences with small groove openings. Using standard roll pass dressing technology, a contact surface with three peaks, which are located in the groove base and at the groove discontinuities, respectively, is created. This is described on p. 1052, FIG. 11 of the aforementioned publication. In very thick-walled tubes with a wall thickness/diameter ratio greater than 0.25, the peaks in the contact surface between the roll and the rolled material again result in internal non-circularities on the tube and thus have a significant negative impact on tube quality. Furthermore, friction is increased, so that the rolls abrade more intensely.